Not Applicable.
Not Applicable.
The present invention relates to methods for producing high purity glasses, in particular, to an improved sol-gel method for preparing powders that can be processed into high purity glasses for use in optical fibers.
Optical communication through the use of glass fibers, commonly known as xe2x80x9cfiber-optic communication,xe2x80x9d has increased dramatically over the past decade. The growing demand for fiber-optic systems has prompted the development of various types of glasses that, when processed into fibers, improve the efficiency, capacity, and functionality of fiber-optic networks. The improved properties are typically obtained by incorporating new elements or compositions into the glass structure. For example, glasses doped with rare-earth elements are being used successfully as xe2x80x9coptical amplifiersxe2x80x9d in fiber-optic systems.
It has been found that conventional methods for preparing fiber optic glasses are often unsuitable for preparing these improved glasses. One problem is that the dopants or other additives tend to cluster at an atomic level within the glass. Rare earth dopants are particularly susceptible to this problem because the elements tend to associate with each other when subjected to high temperatures. Because dopants are usually added to alter the optical properties of the glass, it is desirable to have a more uniform distribution. Another problem is that volatile dopants are vaporized and lost to the atmosphere when high temperature vapor methods, such as chemical vapor deposition, are used.
These problems have been partially addressed by the use of a sol-gel preparation method. As disclosed in U.S. Pat. No. 5,123,940 to DiGiovanni et al., for example, the glass precursors (a metal alkoxide xe2x80x9chostxe2x80x9d material and any dopant materials) are dissolved in a solvent, and the mixture is subsequently hydrolized and condensed into a sol. The sol s applied by dip-coating to the inner surface of a fused silica tube, where it is dried and then the tube collapsed to complete the formation of a glass fiber. Because the glass precursors are applied to the surface of the tube in solution form rather than by flame vapor deposition, volatile dopants are not lost from the system. Sol-gel methods such as that disclosed by DiGiovanni et al. thus allow for the incorporation of a greatly expanded range of elements and compositions into the glass and resulting optical fiber. Furthermore, mixing of the glass precursors on a molecular scale (in the solution) prior to melting results in a more homogeneous glass and more even distribution of dopants. The lower glass-formation temperature characteristic of the sol-gel process also contributes to improved uniformity of dopant distribution. As disclosed in U.S. Pat. No. 5,314,518 to Ito et al., the rare earth elements are not as likely to cluster at lower temperatures.
Despite these clear advantages, the sol-gel method has not gained widespread use for the manufacture of optical glass fiber cores. While dopants are distributed more uniformly than with conventional vapor deposition or batch annealing techniques, clustering still occurs because the dopants aggregate while in solution. A more significant problem with the sol-gel method is that it is very wetxe2x80x94in other words, residual OH groups reside in the sol and the gel. If these OH groups are not removed before the gel is processed into a glass, they are incorporated into the glass structure. The incorporated hydroxide groups cause light having a wavelength of 1390 nm to be absorbed by the doped glass article. The overtone of this OH absorption peak at 1390 nm interferes with the telecommunications transmission wavelength of 1550 nm. As a result, a fiber made from the glass will have poor optical quality for telecommunications applications and will be considered optically impure. A common procedure for removing hydroxide groups involves introducing a chlorine gas flow over the gel as it is sintered into a glass at high temperatures. This technique is described in further detail in U.S. Pat. No. 5,123,940 to DiGiovanni et al. Despite the removal of a significant amount of OH, however, this technique is not effective for reducing OH in the resulting glass fiber to levels adequate for optical communication and amplification applications.
Thus, there is a need for improvements to the sol-gel method of preparing optical fibers. In particular, there exists a need for a sol-gel preparation method that accomplishes uniform distribution of dopants in a substantially hydroxide-free glass, which can subsequently be rendered into fiber form via standard fiber-draw techniques.
Accordingly, it is an object of the present invention to provide a sol-gel method for preparing a substantially hydroxide-free glass.
Another object of the present invention is to provide an improved sol-gel method for preparing a glass, where the glass contains less hydroxide groups than at least some other glasses prepared by conventional sol-gel methods.
An additional object of the present invention is to provide a method for preparing a substantially hydroxide-free glass, where a wide range of elements and compositions can be incorporated into the glass.
Still another object of the present invention is to provide a method for preparing a glass in which dopants are uniformly distributed.
Yet another object of the present invention is to provide a method for preparing a substantially hydroxide-free glass fiber, where a wide range of elements and compositions can be incorporated into, and uniformly distributed in, the fiber.
Other objects of the invention will become apparent to one skilled in the art who has the benefit of the specification and the prior art.
One aspect of the invention which satisfies one or more of the foregoing objects, in whole or in part, is a method that includes the steps of selecting precursor compounds for a glass material, mixing the precursor compounds in a solvent to form a solution, hydrolyzing and condensing the solution, treating the solution with bromine to remove water and hydroxide groups drying the solution until only a powder remains, and heating the powder to remove excess organic material, such that the powder is suitable for processing into a glass.
Another aspect of the invention is a method as previously defined, where at least one of the precursor compounds contains a dopant to be incorporated into the glass.
Still another aspect of the invention is a method as previously defined, where, in addition to a dopant, one of the precursor compounds is a complexing ligand that prevents aggregation of the dopant in the powder and in the resulting glass.
Yet another aspect of the invention is a method as previously defined, where the solvent is water.
Another aspect of the invention is a method as previously defined, where a bromide-containing compound is used to remove water and hydroxide groups from the solution.
Still another aspect of the invention is a method as previously defined, where the solution is hydrolyzed by first adding an acid to the solution, and then by adding a base to the solution.
Another aspect of the invention is a method for forming a glass fiber that includes the steps of preparing a powder by the method as previously defined, dispersing the powder in a solvent to form a suspension, depositing the suspension on the inner surface of a glass tube, and heating the tube to convert the suspension into a glass and to collapse the tube into a glass fiber.